Make CI_COMMIT_REF_SLUG usable as hostname
Description
CI_COMMIT_REF_SLUG provides a shortened and cleaned string to be used in domain names / URLs. There are some use cases for using this slug in hostnames of servers too (when provisioning an EC2 instance for example). Make it possible to use this slug safely as a hostname too.
This is basically already possible, except for one edge case that prevents using the CI_COMMIT_REF_SLUG as a hostname: When the 1st / 63rd character of the CI_COMMIT_REF is a non-character or non-number then the CI_COMMIT_REF_SLUG starts / ends with a hyphen.
According to RFC 952 trailing and leading hyphens are disallowed. Therefore when using CI_COMMIT_REF_SLUG as a hostname there are rare occasions where the hostname is not valid. All the other Properties of the CI_COMMIT_REF_SLUG fulfill the RFC952 regulations on hostnames.
Also a minor point could be made about the aesthetics of trailing / leading hyphens in domain names, but lets not go down that avenue ;)
Proposal
Change the generation of the CI_COMMIT_REF_SLUG to omit leading and trailing hyphens.